The determination of the so-called nozzle opening pressure for injection systems is especially important in order that, in spite of random variations from injector to injector, it is possible to realize in a reproducible manner the precise injection of a small pilot quantity. For example, with piezo-pump nozzle elements the prior art permits the nozzle opening pressure to be set with an accuracy of ±2–5%. Even to realize this precision requires a high expenditure on measurement technology.
The internal pressure within the nozzle works on the so-called pressure stage of the nozzle, which specifies the guide and seat diameter of the injector needle. This internal pressure applies a force which opens the injector needle. The injector spring works against this, to ensure that the injector needle remains reliably closed until the nozzle opening pressure is reached. The nozzle opening pressure is determined by the dimensions of the pressure stage of the nozzle and the rigidity of the injector spring. When the value of the pressure stage has been determined, the nozzle opening pressure can be most precisely set by the choice of the injector spring. This is applicable provided that injector springs are available with adequately fine categorization.
In the case of common-rail systems, the prior art has enabled the nozzle opening pressure to be set without any problem, because the pressure stages are specified with extreme accuracy by reference to their cylindrical geometries; here, the diameters are manufactured with an accuracy of ±3 μm. However, with piezo-pump nozzle elements it can be necessary to change the geometry in the region of the injector needle seat, from a cylindrical seat geometry to non-cylindrical geometries, for example a double-cone geometry. However, for reasons of manufacturing technology, this double-cone can in general only be made to an accuracy of ±50 μm, at great expense within ±20 μm. This is, however, not adequate to guarantee the required accuracy of ±1% in the setting of the nozzle opening pressure.
DE 100 27 181 A1 discloses a method for determining the seat diameter, and hence the pressure stage of the nozzle, but with this the accuracy of measurement lies above ±1%. This is due to the fact, on the one hand, that with this method the injector needle is moved, so that the friction which occurs in the process affects the measurement. On the other hand, shortly before the nozzle opening pressure is reached the seal on the injection nozzle becomes slightly leaky, so that the measured value is slightly falsified. Furthermore, this method is also very sensitive with respect to pressure pulses. For this reason, the pressure must be raised as slowly as possible, which results in a disadvantageous increase in the measurement duration with this method.
Another method which has been disclosed for determining the pressure stage of the injection nozzle consists in making a precise optical or geometric measurement of the seat region. When doing so, however, any rounded edges and other tolerances (e.g. roughness, concentricity, coaxilality, etc.) of the injector needle, but also the measurement accuracy of the systems (measurement accuracies in determining diameters and lengths, optical resolution etc.) and abrasion matters (breakages of the reference edges) have such a strong effect that the accuracy demanded above cannot be achieved. Another possibility consists in vibration testing which is, however, generally used for the determination of the frictional properties. This method could also be modified for the determination of the pressure stage, but the friction which arises in the process would represent a critical interference variable, probably reducing the accuracy too greatly.